Turbochargers are known which intake hot exhaust gases from an internal combustion engine and convert the energy contained within the exhaust gases to kinetic (rotational) energy by means of a turbine wheel mounted rotationally fast to a shaft. The rotational motion of the turbine wheel is transferred along the shaft to rotate a compressor wheel, which draws in and compresses air for delivery to the cylinders of the engine. The use of a turbocharger can considerably improve the overall efficiency of a combustion engine but it is important to keep energy losses in the turbocharger low. It is also an important design consideration for a turbocharger to respond quickly to exhaust gas flow and so reduce turbo lag and the various requirements have to be balanced for any given application.
It is known to use ceramic turbine wheels in a turbocharger. Turbine wheels made of ceramic material are preferred over wheels made of a metallic material, such as Inconel, due to the high temperature strength and relatively low density of ceramic materials. This allows use of the turbine wheel at elevated operating temperatures while the mass moment of inertia is reduced in comparison to an Inconel turbine wheel. Ceramic turbine wheels are lighter, have a higher strength, and are harder and more corrosion resistant than their Inconel equivalents. Use of a ceramic turbine wheel allows turbochargers to be designed which have the following advantages over a turbocharger having an equivalent Inconel turbine wheel:                more rapid response, reducing turbo lag;        ability to work at a lower engine speed, which helps cutting emissions of vehicles in urban area where cars do not move fast but produce high emissions;        ability to run faster, which allows a turbocharger to be reduced in size for a given flow rate;        longer lasting.        
It is common to employ a bearing system to support a rotating shaft, such as the connecting shaft in a turbocharger. In a known arrangement, the bearing system comprises radial sleeve or journal bearings for supporting rotary loads and an axial thrust bearing. In an effort to reduce friction losses and to increase speed of response, it is known to adopt non-contact bearings including fluid bearings and more particularly gas and especially air bearings.
In a gas bearing, a thin film of pressurised gas provides a very low friction load bearing interface between the relatively moving surfaces. There are two main types of gas bearing, gas-static (aero-static) and gas-dynamic (aero-dynamic). In a gas-static bearing, pressurised gas is supplied from an external source, usually a pump, compressor or compressed gas reservoir, to form the load bearing layer. In a gas-dynamic bearing, relative movement between the surfaces of the bearing is used to generate the supportive layer with no external supply of pressurised gas.
Use of gas bearings in a turbocharger offers a number of advantages when compared with an equivalent turbocharger using conventional oil bearings. These include:                more rapid response, reducing turbo lag;        ability to run faster, which allows a turbocharger to be reduced in size for a given flow rate;        increase in output power of the compressor, i.e. more power is recycled back to boost the engine;        cleaner due to absence of oil, no leakage or additional pollution caused by burning oil;        reduction in fuel consumption and emissions, particularly when cars travel at medium speed in urban areas.        
A problem with gas-dynamic bearings is that at low speeds the pressure generated in the layer of gas may be insufficient to support the load, resulting in contact between the parts of the bearing. In this event, friction is increased dramatically reducing the efficiency of the system, but also possibly causing damage through wear and tear of the parts of the bearing contacting each other. Gas-static bearings avoid this problem by using an external source to supply pressurised gas regardless of the speed of rotation. However, known systems for delivering pressurised gas for use in gas-static bearings tend to be rather bulky and so are unsuitable for use in mobile applications such as a motor vehicle, where space and weight limitations are significant. The external source for supplying pressurised gas also consumes energy, reducing the overall efficiency of the system
There is need to provide a gas bearing arrangement which overcomes, or at least mitigates, the problems of the known gas bearings.
There is a need in particular for an improved gas bearing arrangement suitable for use in a turbocharger which overcomes, or at least mitigates, the problems of the known gas bearings and for a turbocharger incorporating such an arrangement.